Web programming – guest lecture in CS640 Cristian Estan November 13 2007
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Web programming
Cristian Estan – guest lecture in CS640
November 13 2007
The web as a platform for applications
Feature
Web app.
Desktop app.
Graphics
Strong
Unlimited
User interaction
Strong
Unlimited
Network usage
High
Varies
Accessible from
Any computer
Where installed
Upgrade cost
Update servers
Update desktop
Data backup cost
Backup servers
Backup desktop
Popularity
Increasing
Dominant
Most programs are event-oriented
A naïve view
Structure of program
Read input
Perform computation
Produce output
The program may use
libraries written by
others
A realistic view
Structure of program
Wait for events
Find appropriate
method for handling
the event
Handle the event
Repeat
Often you just add new
events and handlers to
an existing program
Overview
Web documents
Server-side programming
Client-side programming
Web services
HyperText Markup Language
Disagreement about HTML’s role
Only give the content and structure of the document, leave
visualization to the browser
Browsers vary (graphical, text based, mobile devices)
User preferences vary (some people like larger fonts)
Environment varies (screen sizes, fonts available, etc.)
But authors want to control what the document looks like
Trend towards separating content from presentation
Cascading Style Sheets – presentation information only
HTML documents contain little formatting
Current state of the standards
In the 90s browser wars (IE vs. Netscape) were
driving the evolution of HTML
Non-standard extensions used by pages lead to lock-in
W3C (World Wide Web Consortium) sets standards
Last HTML standard 4.01 (December 1999)
XHTML 1.0 new XML-based format
XML (extensible markup language) – focuses on
semantics and is used as general purpose format for
structured data
A document called DTD or XML Schema defines what
tags and attributes are allowed in an XML document
<TITLE>Bucky Badger’s web page</TITLE>
<BODY>
<H1>Welcome to Bucky's web page</H1>
<IMG SRC="bucky.gif">
<P>I am Bucky, the mascot for University of Wisconsin
athletics. Please visit
<A HREF="http://www.uwbadgers.com/football/index.html">
the web page of our football team</A>
and <A
HREF="http://www.uwbadgers.com/basketball/index.html">
the web page of our basketball team</A>.
</BODY>
A valid web page
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
<title>Bucky Badger’s web page</title>
</head>
<body>
<h1>Welcome to Bucky's web page</h1><!-- Users don’t see this comment. -->
<img src="bucky.gif" alt="A picture of Bucky" />
<p>I am Bucky, the mascot for University of Wisconsin athletics. Please visit
<a href="http://www.uwbadgers.com/football/index.html"> the web page of our
football team</a> and
<a href="http://www.uwbadgers.com/basketball/index.html"> the web page of
our basketball team</a>.</p>
</body>
</html>
About forms
Forms are the traditional way for users to
send information to a web server
The user fills out fields in the browser
The user submits the form
http carries the user input to the web server
A server side program processes the user data
The server sends a reply document to the
client
<h3>Search form</h3>
<form method="get“
action="http://www.googlesyndicatedsearch.com/u/univwisc">
<p>What are you looking for?
<input type="text" name="q" id="searchText" value="Your search terms..." />
<input type="hidden" name="hl" value="en" />
<input type="hidden" name="ie" value="ISO-8859-1" />
<input type="submit" id="searchButton" value="Search UW-Madison" />
</p></form>
Overview
Web documents
Server-side programming
Client-side programming
Web services
Server side programming
Short history
CGI – separate programs launched by web server
They produce an HTML document as output
They receive arguments as input
Strong isolation, bad performance
Programs embedded inside web page (php, ASP, JSP)
Program executed inside web server process
Separate “code-behind” file for the code (ASP.NET)
What are dynamic pages used for?
Personalizing based on user identity
Interacting with databases (e.g. on-line banking)
Web applications (e.g. web based email)
Separate database keeps persistent data
“Lifecycle” of static web page
Web server machine
HTTP request
Server data
URL
Server code
Request
HTML file
Web
client
HTTP response
File
system
Lifecycle of ASP.NET webpage
Web server machine
HTTP request
Server data
Server code
Request
URL
Web
client
.aspx file
codebehind
HTTP response
HTML snippets
Objects representing this web page
File
system
Page with database interaction
Web server machine
HTTP request
Server data
Server code
Request
URL
Web
client
.aspx file
codebehind
HTTP response
Database
SQL interaction HTML snippets
Objects representing this web page
File
system
Trivial ASPX File
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Stub.aspx.cs"
Inherits="Stub" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>Untitled Page</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:Label ID="lblTime" runat="server" />
</div>
</form>
</body>
</html>
Trivial Code Behind File
using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
public partial class Stub : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
lblTime.Text = DateTime.Now.ToLongTimeString();
}
}
Output from Trivial Page
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head><title>
Untitled Page
</title></head>
<body>
<form name="form1" method="post" action="Stub.aspx" id="form1">
<div>
<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="/stuff" />
</div>
<div>
<span id="lblTime">2:52:13 PM</span>
</div>
</form>
</body>
</html>
Overview
Web documents
Server-side programming
Client-side programming
Web services
Why is JavaScript important?
Web pages can contain JavaScript programs executed
inside the browser
Supported by all major browsers
Microsoft’s version called Jscript (the language is the same)
User may disable JavaScript due to security fears
This is default for some newer versions of Internet Explorer
Client-side programming important for web because
Can promptly validate user input
Can update the web page without postback to server
Allows page to react to user actions other than pushing a
“submit” button – more interactivity
What is JavaScript?
Interpreted, object-oriented programming language
with dynamic typing
Introduced by Netscape with Netscape 2.0 in 1995
Standardized as ECMAScript by ECMA (European
Computer Manufacturers Association)
Not related to Java other than the name
Tightly integrated with browser
Can handle many types of events generated by the
normal interaction between user and browser
Can modify the internal objects based on which the
browser renders the web page
<head>
<title>JavaScript Hello world!</title>
</head>
<body>
<script>
document.write("Hello world!");
</script>
</body>
Adding JavaScript to a page
Using the <script> </script> tag
Text between tags is JavaScript program
Can specify external file using src attribute
Executed as the document is loading
Value of an attribute such as onclick
This type of code is called event handler
Executed when event happens
Can define event handlers for almost any
HTML element in page
Some events JavaScript can handle
Handler
Triggered when
Comments
Return false to cancel default action
onclick
Mouse click on element
onchange
The control’s state changes
onfocus
The control gets focus
onsubmit
The form is submitted
onmouseover
Mouse moves over el.
onmouseout
Mouse moves off el.
onmousedown
Mouse button pressed
onmouseup
Mouse button released
onkeydown
Key pressed down
onkeypress
Key pressed and released
Used for form elements and <body>
Return false to cancel
onkeyup
Key released
Used for form elements and <body>
onload
Document load complete
Used for <body> and <img>
Specific to forms
Document Object Model
Describes how the document object from
JavaScript can be traversed and modified
Represented as tree structure
Can add new elements to the page
Can change attributes of existing elements
DOM has levels 0-3 and many sub-standards
The DOM interface used in other contexts with
other languages (C++, Java, python, etc.)
The document as a tree
<html>
<head>
<title>A Document</title>
</head>
<body>
<h1>A web page</h1>
<p>A <i>simple</i>
paragraph</p>
</body>
</html>
document
<html>
<head>
<body>
<title>
<h1>
“A document”
“A web page”
<p>
“A ”
<i>
“simple”
“ paragraph”
Overview
Web documents
Server-side programming
Client-side programming
Web services
What are web services?
A form of remote procedure call: your program (the client) asks
another computer (the server) to run a procedure for you
Parameters sent over the network from client to server
Results sent over network from server to client
Why would you ever want to do a remote procedure call?
Data needed for answer not (easily) accessible to your
computer
You want to re-use existing procedures that run in a different
environment than your program
Your computer lacks the resources (i.e. processor capacity,
memory, network connection speed) to compute the result
There are many other forms of RPC older than web services
CORBA, DCOM, SunRPC, RMI
Internals of an RPC framework
Code for marshalling/unmarshalling – encoding and
decoding parameters/results
A.k.a. serializing objects
Description of the available procedures (methods)
Using an interface description language (IDL)
Framework that turns these descriptions into “stubs”
On the client the stub makes it look to your program
like the stub is executing the procedure locally
On the server the stub invokes the procedure
The client and server stub interact over the network
Specific to web services
They run over http
Procedure call is in an http request
Result is in an http response
They use XML to
Encode responses
Encode requests (sometimes)
Describe the procedures (incl. arguments and results)
Client and server often use different languages
Client may be JavaScript code in browser – AJAX
Client and server are often in different organizations